Present day customer premise data communication systems must be capable of providing service for work stations having diverse types of terminal equipments and differing service needs. These systems must, for example, serve work stations having digital telephones, analog telephones, and various types of data terminal equipment. The data terminal equipment can operate with a variety of interfaces, transmission rates, and transmission protocols. Interconnection must be provided between these various data terminal types and host computers or transmission facilities when they are interconnected by communication switching facilities.
In prior art arrangements, a switching system could provide both voice and/or data service only by the provision of customized equipment preselected for each type of device. The phone service was customarily provided by a dedicated tip and ring analog pair. The data service was provided by the use of a second tip and ring pair together with an appropriate modem for digital to analog conversion.
Data terminals operate at different transmission rates and formats as determined by the needs of each customer and the characteristics of the available communication channels. When only analog channels are available, the types of modems available fell into two general classes--asynchronous or synchronous.
Asynchronous modems support data transmission at relatively low rates (generally .ltoreq.1200 bits per second). Asynchronous data transmission is characterized by character by character operation with each character framed by start and stop bits. The asynchronous modem, while not providing particularly high transmission rates, is inexpensive to implement and the individual character framing allows simple terminal interface protocols.
Synchronous data transmission is characterized by a continuous data stream together with a bit synchronized clock. Synchronous modems take advantage of the bit to bit coherence of the transmitted data stream to operate at relatively higher rates (generally .ltoreq.9600 bits per second). Since there is no character framing inherent in the synchronous data transmission strategy, mechanisms have been developed to frame blocks of characters and provide data transparency. Transparency means that the transmitted data patterns are not constrained by the communication strategy. These framing mechanisms, together with strategies for error and flow control, link initialization, etc., make up what are generally known as link level protocols. Many have been developed by different terminal or computer manufacturers or standards bodies (e.g. HDLC, BISYNC, DDCMP).
These synchronous protocols are generally much more complex and therefore more expensive to support than the simple asynchronous protocols. The principal advantages in using synchronous transmission are higher transmission rates and better performance (e.g. lower error rates). Synchronous modems are transparent to the particular protocol being used. They transmit the synchronous bit stream without knowing any details of the protocol. Because they operate at higher rates, synchronous modems are generally more expensive than asynchronous modems.
In addition to modems and terminal equipment being classed as either synchronous or asynchronous, they can be classed as either half- or full-duplex. Half-duplex operation involves transmission in one direction at a time from device A to device B; then the communication line is "turned around" for transmission from device B to device A. This allows better utilization of the channel bandwidth for communication in one direction at a time.
Full-duplex operation allows both devices on the communication link to transmit to the other simultaneously. For analog channels, full-duplex generally reduces the bit rate in each direction.
In order to control analog modems, interface standards such as RS232C evolved. This standard of the Electronic Industries Association, as well as others, provides a multiwire (25 lead) interface definition between data terminal equipment and modems. Interface leads were defined for data and timing, as well as multiple control leads to control the state of the modem and the direction of transmission (for half-duplex modems). The state on one modem is often transferred indirectly to the modem at the other end of the connection to affect the state of the other modem. For example, a Request-to-Send control lead from a terminal to a half-duplex modem causes the modem to begin sending a carrier signal to the receiving modem. The receiving modem, upon detection of the carrier, will set its Carrier Detect control lead true. An analog modem pair, therefore, transmits data, timing, and control information between two pieces of data equipment (terminals or computers).
In summary, the present day data communication environment contains different types of data terminal equipment using a variety of transmission rates, formats, protocols, and control strategies. Thus, even though present day switching systems can be equipped to serve stations having phones and/or terminals, the provision of this service requires customized equipment at each station to serve the specific characteristics of that equipment and the communication channels available.
It is therefore a problem for present day communication switching systems to provide voice plus data service with the use of universal station interface equipment that is ubiquitously adapted to serve a wide spectrum of station equipments.